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Dispelling two pesky car myths

Although the price of gasoline is heading downward to levels that are merely outrageous rather than insane, consumers are still rightly concerned about various issues.

Here are two questions raised by readers that cover important issues worth explaining in greater depth:

Does driving on 'empty' hurt the engine?

q_v2.gifDear Terry,
With gas as expensive as it is, I've been driving my car until the gas light comes on. But this goes against everything my dad taught me.

He said to never let your gas go below a quarter of a tank, otherwise you could get dirt in your engine and ruin your car.

Is he right, or does he just not want me to run out of gas?
-- Riding Dirty

a_v2.gifDear Riding,
Dear old Dad may have been right about many things, but when it comes to modern vehicles, he has this one mostly wrong.

Almost all cars have their fuel pumps installed inside the gas tank and the pump picks up the fuel from the lowest part of the tank, so you can get almost every last drop from the tank.

If there is a lot of dirt and debris -- say, from a corroding fuel tank -- that gunk would normally settle to the bottom and be picked up by the fuel pump regardless of how much fuel was in the tank.

But modern fuel pumps have a protective screen or porous sock-like cover that catches any contaminants before they enter the fuel system. And those small bits that might get through would normally be caught by a second filter closer to the engine.

Back in the day, when manufacturers were churning out cars with very little rust protection, it was possible that after four or five years there could be some corrosion on the inside of the gas tank that could result in sediment. But most gas tanks these days are not prone to such issues.

Where your dad may have been right is that you should always try to never run out of gas. That fuel pump inside the gas tank relies of the gasoline to keep it lubricated and cool, and if you frequently run out of gas you could cause the pump to fail.

The good news is that the "low fuel" light comes on when there's anywhere from 1 to 2 gallons left, which is plenty to keep the pump safe.

So it's OK to run the tank down to where the warning light comes on, but you should then get gas as soon as possible.

Does shutting down in traffic save gas?

q_v2.gifDear Terry,
When is it worth it to turn off your car to save gas? A draw bridge near my home has a sign that says to turn off the engine while bridge is up, but will it waste gas to start up the car after the bridge goes down? And what about turning it off in drive-throughs or when waiting at the bank?
-- Fussy Fuels-a-lot

a_v2.gifDear Fussy,
This is a lot like the question of whether you should turn off the light in a room when you leave for just a few minutes. The notion is that turning the light back on may require a surge of electricity greater than the electricity the light would consume if left burning.

I can't speak to the light question, but there are times when shutting down your car while idling will save gasoline.

First, all vehicles built since about 1990 have electronic fuel injection that much more efficiently doles out the gas to the engine. When you start today's cars, there's not a big rush of fuel to a carburetor.

So anytime it looks like you're going to be idling for more than 30 seconds, you can save a bit by turning off the engine.

Some practitioners known as "hypermilers" turn off the engine at every stop light, then quickly turn it back on when the light goes green. They claim fuel mileage increases of 30 percent or more, but I think that's a lot of work and perhaps imparts greater wear on the starter mechanisms and the transmission, which could result in eventual repairs that would outweigh any savings at the pump.

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Also, if you live in a warm climate, you make find that going without air conditioning while the engine is stopped is not worth the savings.

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